I would like to transfer/convert my existing Gliffy diagrams to draw.io, is it free to do so?
Yes, the transfer is free.
Switch from Gliffy to draw.io
You can gain more knowledge about why you should switch from Gliffy to draw.io. You can read your way through the advantages regarding usability and economic benefits for the license.
Comparing draw.io, Gliffy and Lucidchart
An additional comparison of the three popular diagram tools draw.io, Gliffy and Lucidchart can be also helpful.
Weblog and general information about draw.io for Confluence
There is also a weblog under https://drawio.seibert-media.com/. We can also give you some general information and detailed documentation for draw.io for Confluence.